Differences in Intraseasonal Summer Rainfall Oscillation between the Middle and Lower Reaches of the Yangtze River
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摘要: 利用1979~2013年中国站点逐日降水资料和NCEP/NCAR再分析资料，对长江中下游夏季降水的季节内振荡最显著周期进行了分析研究。结果表明长江中游最显著周期为10~30天，长江下游最显著周期为30~60天。为了揭示这种差异产生的物理原因，进一步利用位相合成的方法对这两个区域不同周期的季节内振荡降水、高低空风场和高度场以及垂直结构和水汽等循环过程的演变特征进行分析。在200 hPa环流场上，长江中游的降水主要受到高纬度自西向东传播的波列影响，而长江下游的降水与鄂霍次克海的高度场的变化相关。在风场的垂直涡度和散度的位相结构演变过程中，10~30天的垂直涡度和散度有自北向南的移动，30~60天的垂直涡度和散度在长江以南地区有自南向北的传播。水汽输送的位相发展过程表明，长江中游的水汽分别来自于南海的向北输送和长江以北地区向南的水汽输送；长江下游地区的水汽则主要来自于热带东印度洋经孟加拉湾的向东输送并在南海的北向输送，以及西太平洋水汽向西输送到南海再向长江下游的输送。从高层大尺度环流场和整层积分的水汽通量输送上解释了长江中游10~30天降水的自北向南移动，和长江下游30~60天降水自南向北传播的原因。Abstract: The most significant climatological intraseasonal oscillation (ISO) periods of summer rainfall over the middle and lower reaches of the Yangtze River (M-LYR) are investigated by using the gauge-based daily precipitation analysis data of China and the NCAR/NCEP reanalysis data from 1979 to 2013. It is found that the oscillation periods of summer rainfall at the intraseasonal timescale show remarkable difference between the MYR and LYR. The 10-30-day oscillation is the primary mode over the MYR, while the longer period of 30-60-day dominates the LYR. The phase composite analysis is applied to reveal evolutions of intraseasonal circulation and vertical structure associated with these two periods of climatological ISO precipitation over the Yangtze River. In the upper troposphere at 200 hPa, it shows that a wave train over the mid-high latitudes propagates eastward, affecting the summer rainfall over MYR, while the southwestward propagation of the weakened Okhotsk blocking high is associated with the 30-60-day rainfall oscillation over LYR. The vertical structures of wind vorticity and divergence/convergence exhibit different features during their moving toward the Yangtze River. 10-30-day anomalous vorticity and divergence/convergence move southward toward the MYR; however, the 30-60-day anomalies move northward from the tropics. Atmospheric moisture from the South China Sea and from north of the Yangtze River converges over MYR, leading to favorable condition for the occurrence of rainfall. In contrast, the moisture in the LYR primarily comes from the tropics. Large amounts of atmospheric moisture originate from the eastern Indian Ocean, move through the Bay of Bengal and the South China Sea, converge with moisture transported westward from the western tropical Pacific, and then move northward, reaching the LYR. The results explain from the perspective of upper-level circulation and vertically integrated moisture flux why the 10-30-day oscillation mode of rainfall over MYR propagates southward, while the 30-60-day oscillation mode of rainfall over LYR propagates northward.
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图 1 1979~2013年长江中下游区域夏季降水季节内振荡最显著周期分布，其中红色方框区域为本文选取的长江中游（27°~32°N，106°~115°E）和长江下游地区（27°~32°N，115°~123°E）
Figure 1. Distribution of the most significant intraseasonal oscillation periods of summer rainfall over the middle reaches of the Yangtze River (MYR) (the left red box; 27°–32°N, 106°–115°E) and the lower reaches of the Yangtze River (LYR) (the right red box; 27°–32°N, 115°–123°E)
图 2 长江中游季节内尺度降水序列的小波图：（a）逐日季节内尺度降水序列；（b）季节内尺度降水序列的小波分析频谱；（c）小波全谱
Figure 2. Wavelet spectra of the time series of intraseasonal rainfall over the MYR: (a) Time series of the ISO (Intraseasonal Oscillation) rainfall; (b) wavelet power spectrum of intraseasonal rainfall; (c) full wavelet spectrum
图 3 长江下游季节内尺度降水序列的小波图：（a）逐日季节内尺度降水序列；（b）季节内尺度降水序列的小波分析频谱；（c）小波全谱
Figure 3. Wavelet spectra of the time series of intraseasonal rainfall over the LYR: (a) Time series of the ISO rainfall; (b) wavelet power spectrum of intraseasonal precipitation; (c) full wavelet spectrum
图 4 1979~2013年5~10月降水原始序列（直方图，单位：mm）和标准化的滤波序列（红色曲线，单位：mm）：（a）长江中游地区（10~30天滤波）；（b）长江下游地区（30~60天滤波）。黑线为正/负1倍标准差，数值表示准双周振荡位相
Figure 4. Time series of daily rainfall in summer (gray bars; units: mm) and standardized series of the filtered rainfall anomaly (red solid line; units: mm): (a) Over the MYR (10–30-day filtered); (b) over the LYR from 1979 to 2013 (30–60-day filtered). Black lines represent±1.0 times of standard deviation of the ISO rainfall, numbers represent the phase of the biweekly oscillation
图 5 10~30天夏季降水的各位相合成（单位：mm d-1），黑色方框代表长江中游地区
Figure 5. Composite 10–30-day filtered summer precipitation (units: mm d-1). The black box represents the MYR region
图 6 30~60天夏季降水的各位相合成（单位：mm d-1），黑色方框代表长江下游地区
Figure 6. Composite 30–60-day filtered summer precipitation (units: mm d-1). The black box represents the LYR region
图 7 长江中游（红色方框）降水10~30天振荡各位相合成的850 hPa低频位势高度场（阴影，单位：gpm）与850 hPa低频风场（矢量，单位：m s-1），只画出大于0.1 m s-1的风矢量，黑线代表青藏高原高度大于3000 m的部分
Figure 7. Composite 10–30-day filtered 850 hPa geopotential height (shading; units: gpm) and wind (vectors; units: m s-1) over the MYR (the red box). Only wind speeds larger than 0.1 m s-1 are shown. The black contour indicates the Tibet Plateau region with elevations exceeding 3000 m
图 8 长江中游（红色方框）降水10~30天振荡各位相合成的200 hPa的低频位势高度场（阴影，单位：gpm）与低频风场（矢量，单位：m s-1），只画出大于0.2 m s-1的风矢量
Figure 8. Composite 10–30-day filtered 200 hPa geopotential height (shading; units: gpm) and wins (vectors; units: m s-1) over the MYR (the red box). Only wind speeds larger than 0.2 m s-1 are shown
图 9 长江下游（红色方框）降水30~60天振荡各位相合成的850 hPa的低频位势高度场（阴影，单位：gpm）与低频风场（矢量，单位：m s-1），只画出大于0.15 m s-1的风矢量，黑线代表青藏高原高度大于3000 m的部分
Figure 9. Composite 30–60-day filtered 850 hPa geopotential height (shading; units: gpm) and wind (vectors; units: m s-1) over the LYR (the red box). Only wind speeds larger than 0.15 m s-1 are shown. The black contour indicates the Tibet Plateau region with elevations exceeding 3000 m
图 10 长江下游（红色方框）降水30~60天振荡各位相合成的200 hPa的低频位势高度场（阴影，单位：gpm）与低频风场（矢量，单位：m s-1），只画出大于0.2 m s-1的风矢量
Figure 10. Composite 30–60-day filtered 200 hPa geopotential height (shading; units: gpm) and wind (vectors; unit: m s-1) over the LYR (the red box). Wind speeds larger than 0.2 m s-1 are shown
图 11 经过（106°~115°E平均）剖面的10~30天滤波的涡度场（阴影，单位：10-6 s-1）和散度场（等值线，实线为正值，虚线为负值，单位：10-6 s-1，等值线间隔为10-7 s-1）的各位相合成
Figure 11. Composite vertical cross sections of 10–30-day filtered vorticity (shading; units: 10-6 s-1) and divergence (contours; solid lines represent divergence and dotted lines represent convergence; units: 10-6 s-1.The contour interval is 10-7 s-1.) averaged over the region of 106°–115°E
图 12 经过（115°~123°E平均）剖面的30~60天滤波的涡度场（阴影，单位：10-6 s-1）和散度场（等值线，实线为正值，虚线为负值，单位：10-6 s-1，等值线间隔为10-7 s-1）的各位相合成
Figure 12. Composite vertical cross sections of 30–60-day filtered vorticity (shading; units: 10-6 s-1) and divergence (contours; solid lines represent divergence and dotted lines represent convergence; units: 10-6 s-1.The contour interval is 10-7 s-1.) averaged over the region of 115°–123°E
图 13 位相合成的整层积分的低频水汽通量输送（矢量，单位：kg s-1 cm-1）及其散度（阴影，单位：10-5 kg s-1 cm-2）：（a、b）长江中游地区；（c、d）长江下游地区。红色方框表示所讨论区域；只画出大于图右上角标值10%的水汽通量输送矢量
Figure 13. Composite vertically integrated moisture fluxes (vectors; units: kg s-1 cm-1) and their divergence (shading; units: kg s-1 cm-1): (a, b) Over the MYR region (the red box); (c, d) over the LYR region (the red box). Integrated moisture fluxes larger than 10% of the vector scale are shown
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